The commercialization of Li-ion battery (LIB) during the last decades has been a major success in secondary battery industry, which made it possible to implement small electronic gadgets into daily life. However, expending the use of Li-ion based battery to larger applications such as electricity storage grid support may be hindered due to the geographical isolation and availability of Li sources.
In this regard, Na-ion battery (NIB) is vastly considered as an excellent alternative to LIB for stationary applications because Na is one of the most abundant elements in earth curst and seawater, and it is also the second lightest alkali metal next to Li. In fact, Na intercalation chemistry of some cathode materials for NIB has been demonstrated during 1980's and recent research progress in electrode materials has shown the feasibility of cost effective fully functional unit that can be implemented into real applications in near future [Palomares, V., Energ. Environ. Sci., 2013, 6, 2312]. It is widely postulated that the energy density of NIB cathode materials may not be able to match that of LIB because Na is more than three times heavier and the average operating voltage is lower than that of Li hosting counterparts. However, despite such postulation, NIB has become an attractive system due to the cost and durability for a long term operation, technical features considered to be more important than the energy density of each unit for large stationary applications.
Among many cathode candidates, layered oxides NaxMTO2 (MT=Ti, V, Cr, Mn, Fe, Co, Ni and combination of two or more thereof) are of great interest because of their large specific capacity, ease of synthesis, and choice of several metal constituents [Han, M. H. et al., Energ. Environ. Sci., 2015, 8, 81].
As an example, P2-phase NaxMnO2 systems have been tested as positive electrodes [Mendiboure, A., et al., J. Solid State Chem., 1985, 57, 323; Caballero, A. et al., J. Mater. Chem., 2002, 12, 1142; Ma, X. et al., J. Electrochem. Soc., 2011, 158, A1307; Su, D. et al., Chem. Eur. J., 2013, 19, 10884]. However, although the electrode delivers a high specific capacity, the charge/discharge profile exhibits multiple plateaus which implies structural instability during the cycling, and, additionally, a rapid capacity fades is also observed, particularly over 7% capacity is faded during only 10 cycles.
α-NaFeO2 exhibits high, flat voltage profile during the cycling but undergoes an irreversible structural change above approximately 3.6 V which causes a rapid capacity degradation after only a few cycles [Yabuuchi, N. et al., J. Mater. Chem., A, 2014, 40, 16851-16855].
Furthermore, a series of Fe and Mn containing layer oxides have been investigated extensively due to the environmental benignity, inexpensive metal constituents, and large reversible capacity. For example, a cathode material having a layer of NaxFe1/2Mn1/2O2 has been disclosed [Yabuuchi, N. et al., Nat. Mater., 2012, 11, 512]. This cathode material does not contain scarce or toxic elements and it showed high reversible capacity in Na-ion batteries. The Fe—Mn-oxide host structure remains intact during sodium de-intercalation and re-insertion. Nevertheless, the cooperative effect of large substitution of transition metals into Mn containing layered oxides has been demonstrated either to attain structural stability at an expense of specific capacity or to have a large specific capacity at an expense of structural stability.
Recently, small substitution of electrochemically inactive element, such as Co and Mg, into Mn-rich P2-phase compound has also been described [WO2014/132174; Billaud, J. et al., Energy Environ. Sci., 2014, 7, 1387-1391 Yabuuchi, N. et al., J. Mater. Chem., A, 2014, 40, 16851-16855]. However, an appropriate balance between specific capacity and cycling performance cannot be achieved and, thus, it still remains as an obstacle for these materials to be used as cathode in Na-ion batteries. In particular, small substitution of Co (˜10%) delivers a specific capacity over 150 mAh/g, but the capacity fades down to 100 mAh/g only after 30 or less cycles. A small substitution of Mg (˜20%) delivers ˜150 mAh/g and a capacity retention of 95% but only for 25 cycles.
Other documents of the prior art describes sodium-ion secondary batteries comprising a positive electrode active material which includes sodium and lithium NaaLibMxO2, wherein M is Mn, Fe, Co, Ni or a combination of two of them [US2007/0218361; US2010/0248040 and US2011/0200879].
Another class of known sodium intercalation cathode materials are phosphates like NaMn0.5Fe0.5PO4, or fluorite-phosphates like NaVPO4F or Na2FePO4F. However, both phosphates and fluorophosphates exhibit certain difficulties in their chemical preparation and have a low gravimetric capacity and their use in Na-ion batteries is quite limited.
In spite of the different cathode materials described in the prior art, there is still much interest in developing new cathode materials having a good balance of properties, in particular, a considerably improved reversible capacity as well as a higher cycle stability so as the electrode can be recharged multiple times without significant loss in charge capacity. Furthermore, it is also desirable for such materials to be straightforward to manufacture at low cost and easy to handle and store.